Algebraic determination of the structure function of Dynamic Fault Trees

This paper presents an algebraic framework allowing to algebraically model dynamic gates and determine the structure function of any Dynamic Fault Tree (DFT). This structure function can then be exploited to perform both the qualitative and quantitative analysis of DFTs directly, even though this latter aspect is not detailed in this paper. We illustrate our approach on a DFT example from the literature.     

The Event Sequence Diagram framework for dynamic Probabilistic Risk Assessment

Dynamic methodologies have become fairly established in academia. Their superiority over classical methods like Event Tree/Fault Tree techniques has been demonstrated. Despite this, dynamic methodologies have not enjoyed the support of the industry. One of the primary reasons for the lack of acceptance in the industry is that there is no easy way to qualitatively represent dynamic scenarios. This paper proposes to extend current Event Sequence Diagrams (ESDs) to allow modeling of dynamic situations. Under the proposed ESD representation, ESDs can be used in combination with dynamic methodology computational algorithms which will solve the underlying probabilistic dynamics equations. Once engineers are able to translate their knowledge of the system dynamics and accident evolution into simple ESDs, usage of dynamic methodologies will become more popular.     

Quantitative Analysis of Dynamic Fault Trees Based on the Coupling of Structure Functions and Monte Carlo Simulation

This paper focuses on the quantitative analysis of Dynamic Fault Trees (DFTs) by means of Monte Carlo simulation. In a previous article, we defined an algebraic framework allowing to determine the structure function of DFTs. We exploit this structure function and the minimal cut sequences that it allows to determine, to know the failure mode configuration of the system, which is an input of Monte Carlo simulation. We show that the results obtained are in good accordance with theoretical results and that some results, such as importance measures and sensitivity indexes, are not provided by common quantitative analysis and yet interesting. We finally illustrate our approach on a DFT example from the literature. Copyright © 2014 John Wiley & Sons, Ltd.     

Application of the integrated reliability analysis system (IRAS)

This paper introduces a UNIX-based computer aided reliability assessment system, IRAS, which was developed in the Brite/Euram project BE-4250. It utilises fault propagation models for automatic generation of Fault Trees, Cause–Consequence Diagrams and FMECA. Therefore, it has the following features: a Model Builder which allows the creation of the fault propagation models in a hierarchical manner; a Fault Tree Analysis module that is able to generate Fault Trees on demand and to extract minimal cut sets; an FMECA module that is able to search for and group effects of basic events according to their criticality, severity and probability; a Real Time Fault Location (RTFL) module that enables the fast detection of the most probable cause(s) of system malfunction based on information available from sensors and/or operator. This paper describes the underlying ideas and procedures of IRAS and shows an example application to a Hot Strip Steel Mill.     

On the numerical solution of fault trees

In this paper an account will be given of the numerical solution of the logic trees directly extracted from the Recursive Operability Analysis. Particular attention will be devoted to the use of the NOT and INH logic gates for correct logical representation of Fault Trees prior to their quantitative resolution.The NOT gate is needed for correct logical representation of events when both non-intervention and correct intervention of a protective system may lead to a Top Event.The INH gate must be used to correctly represent the time link between two events that are both necessary, but must occur in sequence. Some numerical examples will be employed to show both the correct identification of the events entering the INH gates and how use of the AND gate instead of the INH gate leads to overestimation of the probability of occurrence of a Top Event.     

The computer modeling of lanthanum in the framework of faujasite: Part 2. Hydrated lanthanum zeolite Y

The methodology of Part 1 was used to compute the lattice energies of various structures for hydrated La-ZY. The structure deduced by Cheetham et al. from their 5 K neutron diffraction measurements is shown to be compatible with the potentials used to a high degree of consistency, except for framework hydrogen. However, the modeling studies suggest this is not the only feasible or even the most stable structure type. One variant is the same as the diffraction structure except that the H's of the OH⁻'s relate differently to the framework. Much more common among the simulated structures, however, is an arrangement in which one of the extraframework H's is on an SII' site, where it interacts closely with three O2's, and the other is more distant from the framework. The La³⁺ ions remain on SI' sites. An examination of the Madelung and short-range energies of all the species composing the various structures shows that their relative stabilities cannot be understood in terms of a few predominant local interactions. This study also indicates that the conclusions of Cheetham et al. concerning framework hydrogen need to be reviewed.     

Coherence region of the Priority‐AND gate: Analytical and numerical examples

In recent years, the need for a more accurate dependability modelling (encompassing reliability, availability, maintenance, and safety) has favoured the emergence of novel dynamic dependability techniques able to account for temporal and stochastic dependencies of a system. One of the most successful and widely used methods is Dynamic Fault Tree that, with the introduction of the dynamic gates, enables the analysis of dynamic failure logic systems such as fault‐tolerant or reconfigurable systems. Among the dynamic gates, Priority‐AND (PAND) is one of the most frequently used gates for the specification and analysis of event sequences. Despite the numerous modelling contributions addressing the resolution of the PAND gate, its failure logic and the consequences for the coherence behaviour of the system need to be examined to understand its effects for engineering decision‐making scenarios including design optimization and sensitivity analysis. Accordingly, the aim of this short communication is to analyse the coherence region of the PAND gate so as to determine the coherence bounds and improve the efficacy of the dynamic dependability modelling process.     

Bifurcations and fundamental error bounds for fault-tolerant computations

In the emerging nanotechnologies, faulty components may be an integral part of a system. For the system to be reliable, the error of the building blocks has to be smaller than a threshold. Therefore, finding exact error thresholds for noisy gates is one of the most challenging problems in fault-tolerant computations. Under the von Neumann's probabilistic computing framework, we show that computation by circuits built out of noisy NAND gates with an arbitrary number of K inputs under worst case operation can be readily described by nonlinear discrete maps. Bifurcation analysis of such maps naturally gives the exact error thresholds above which no reliable computation is possible. It is further shown that the maximum threshold value for a K-input NAND gate is obtained when K=5. This implies that if one chooses NAND gate as basic building blocks, then the optimal number of inputs for the NAND gate may be very different from the conventional value of 2. The analysis technique generalizes to other types of gates and circuits that use voting to improve reliability, as well as a network built out of the so-called para-restituted NAND gates recently proposed by Sadek et al. Nonlinear dynamics theory offers an interesting perspective to study rich nonlinear interactions among faulty components and design nanoscale fault-tolerant architectures.     

Proposal for optical parity state re-encoder

We propose a re-encoder to generate a refreshed parity encoded state from an existing parity encoded state. This is the simplest case of the scheme by Gilchrist et al. [Phys. Rev. A 2007 75, 052328]. We give a detailed experimental arrangement for the re-encoder and show that it is possible to experimentally demonstrate with existing technology parity encoded quantum gates and teleportation.     

Bullwhip and anti-bullwhip effects in a supply chain

The anti-bullwhip effect, coined by Li et al. in 2005 based on the findings in a simulation study, is the contrary effect of the well-known bullwhip effect. Although there is ample empirical evidence that suggests both effects exist, current literature has not yet provided an integrated framework to address how the two effects are related with each other. By extending the classic work of Lee, Padmanabhan, and Whang of 1997 to a multi-stage supply chain, we derive closed-form formula to analytically describe how the two effects originate initially and then evolve over time and space in the supply chain. Our results show both the bullwhip effect and the anti-bullwhip effect can occur when facing different end-customer demands. However, the magnitude of these effects gradually decreases when moving upstream. We also show the impact of long lead-time on increasing the magnitude of information transformation at the next stage and simultaneously decreasing the magnitude at higher stages. These analytical results provide a theoretical explanation to most simulation findings of Li et al.'s 2005 work and can be used by researchers and practitioners to examine the empirical data and design innovate marketing strategies to convert the unwelcome bullwhip effect into the anti-bullwhip effect.     

Electronic Properties of a New Structure of BC2N and Its Intercalation with Magnesium: Possibility for a New High-Tc Superconductor

We propose a new structure for the graphite-like BC₂N, other than the one proposed in Liu et al. (Phys. Rev. B 39, 1760, 1989). We compare it with the older structure and show that it has a lower energy. The band structure calculations of a single sheet of this new structure show that it is a semiconductor with a very small band gap of 0.25 eV, whereas an AA stacking of BC₂N layers of this structure behaves like a metal. Because of the similarity of this structure with the boron layers in MgB₂, we propose to intercalate the layers of the new structure of BC₂N with the magnesium atoms to obtain Mg₂BC₂N. The band structure calculations of this new structure show an unusually large metallic density of states at the Fermi level, much higher than that of MgB₂. This leads us to expect Mg₂BC₂N to be a superconductor with a higher T _c.     

Probabilistic assessments in relationship with safety integrity levels by using Fault Trees

In this article, we study the assessment of safety integrity levels of safety-instrumented system by means of Fault Trees. We focus on functions with a low demand rate. For these functions, the appropriate measure of performance is the so-called probability of failure on demand (PFD) or probability of not functioning on demand. In order to calculate accurately the average PFD as per IEC 61508 standard, we introduce distributions for periodically tested components into Fault Tree models. We point out the specific problems raised by the assessment of safety integrity levels (SILs), which restrict the use of the formulae proposed in the standard. Among these problems there is the fact that SIL should be assessed by considering the time-dependent behavior of the system unavailability in addition to its average value. We check, on a simple pressure protection system, the results obtained by means of the Fault Tree approach against those obtained by means of stochastic Petri nets with predicates.     

Electronic Properties of a New Structure of BC2N and Its Intercalation with Magnesium: Possibility for a New High-Tc Superconductor

We propose a new structure for the graphite-like BC₂N, other than the one proposed in Liu et al. (Phys. Rev. B 39, 1760, 1989). We compare it with the older structure and show that it has a lower energy. The band structure calculations of a single sheet of this new structure show that it is a semiconductor with a very small band gap of 0.25 eV, whereas an AA stacking of BC₂N layers of this structure behaves like a metal. Because of the similarity of this structure with the boron layers in MgB₂, we propose to intercalate the layers of the new structure of BC₂N with the magnesium atoms to obtain Mg₂BC₂N. The band structure calculations of this new structure show an unusually large metallic density of states at the Fermi level, much higher than that of MgB₂. This leads us to expect Mg₂BC₂N to be a superconductor with a higher T _c.     

Penetration into concrete by truncated projectiles

In this paper, the penetration into semi-infinite concrete targets by rigid truncated projectiles is examined. The semi-analytical cavity expansion based penetration formula in Forrestal et al. (Int. J. Impact Eng. 15(4) (1994) 395) and Frew et al. (Int. J. Impact Eng. 21(6) (1998) 489) for ogive-nosed projectiles has earlier been seen to give good agreement with experiments. In Lixin et al. (Int. J. Impact Eng. 24 (2000) 947), this model was extended to truncated-nosed projectiles by introducing two additional empirical constants. In the present paper, Forrestal's approach is expanded to include a larger class of projectile nose geometries, especially flat and truncated noses. The new theory presented here introduces no new empirical constants, and is seen to agree very well with the available experimental data.     

USAD: undetectable steganographic approach in DCT domain

In this work, we propose a new adaptive chaotic steganographic method based on the Discrete Cosine Transform (DCT) and a reversible mapping function. The mapping function is used to map the secret bits into their corresponding symbols. This mapping technique has to preserve the same dynamics, properties and distribution of the original DCT coefficients. The novelty of our approach is based on the adaptive selection phase of embedding spots. This selection is established through a blindness condition which is applied over each image of the database. The proposed embedding scheme within the middle DCT coefficients shows lower probability of detection and higher flexibility in extraction. We evaluate the detection of our method using the Ensemble Classifiers and a set of frequency and spatial domain feature extractors such as the Spatial domain Rich Model (SRM) features, Chen et al.'s 486-dimensional both inter- and intra-block Markov-based features and Liu's 216-dimensional adaptive steganography-based features.     

Is there a liquid phase in a (Sub)monolayer of3He adsorbed on graphite?

Variational Monte Carlo calculations on two dimensional³He adsorbed on graphite show that the uniform fluid phase is unstable with respect to a self bound fluid. Taking into account the delocalisation of the³He atoms perpendicular to the substrate is essential to obtain this self-bound state. We discuss the likely consequences of this result on the phase diagram of this system. Heat capacity measurements of Greywall et al. and spin susceptibility measurements of Sounders et al. are discussed in the light of this new finding. Explanations of some anomalies and unexplained features of the experimental results are proposed.     

Modelling biological behaviours with the unified modelling language: an immunological case study and critique

We present a framework to assist the diagrammatic modelling of complex biological systems using the unified modelling language (UML). The framework comprises three levels of modelling, ranging in scope from the dynamics of individual model entities to system-level emergent properties. By way of an immunological case study of the mouse disease experimental autoimmune encephalomyelitis, we show how the framework can be used to produce models that capture and communicate the biological system, detailing how biological entities, interactions and behaviours lead to higher-level emergent properties observed in the real world. We demonstrate how the UML can be successfully applied within our framework, and provide a critique of UML''s ability to capture concepts fundamental to immunology and biology more generally. We show how specialized, well-explained diagrams with less formal semantics can be used where no suitable UML formalism exists. We highlight UML''s lack of expressive ability concerning cyclic feedbacks in cellular networks, and the compounding concurrency arising from huge numbers of stochastic, interacting agents. To compensate for this, we propose several additional relationships for expressing these concepts in UML''s activity diagram. We also demonstrate the ambiguous nature of class diagrams when applied to complex biology, and question their utility in modelling such dynamic systems. Models created through our framework are non-executable, and expressly free of simulation implementation concerns. They are a valuable complement and precursor to simulation specifications and implementations, focusing purely on thoroughly exploring the biology, recording hypotheses and assumptions, and serve as a communication medium detailing exactly how a simulation relates to the real biology.     

Singular-hyperbolic sets and topological dimension

A 'singular-hyperbolic set' for flows is a partiallyhy perbolic set with singularities (hyperbolic ones) and volume expanding central direction (Morales et al. 1998, Comptes Rendus de L' Academic des Sciences Paris-Serie I-Mathematics, 326: 81–86). The class of transitive singular-hyperbolic sets includes the geometric Lorenz attractor and the singular horseshoe (Guckenheimer and Williams 1979, inst. Hautes E' tudes Sci Publ. Math., 50: 59-72, Labarca and Pacifico 1986, Topology, 25: 337-352). We prove that all compact, non-singular, invariant subsets of a transitive singular-hyperbolic set are 1-dimensional. This generalizes the minimal set's results for Axiom A flows in Bowen (1973, America Journal of Mathematics, 95: 429-460) to a class of flows studied in Morales et al. (1999, Proceedings of the American Mathematical Society, 127: 3393-3401).     

Superconductivity and Phase Diagram in a CuO2 Monolayer

We recall the van Hove scenario (J. Labbe and J. Bok, Europhys. Lett. 3, 1225 (1987); J. Bouvier and J. Bok, in The Gap Symmetry and Fluctuations in HTSC, J. Bok et al., eds. (Plenum, New York, 1998)) developed since 1987. It explains high T _c, anomalous isotope effect, gap anisotropy etc. We apply this scenario to the superconductive surface layer, obtained by field effect on CaCuO₂ by J. H. Schön et al. Preprint (private communication), to be published). We show that the variation of resistivity and Hall effect with temperature in the normal state can be understood by the presence of a van Hove singularity (v.H.s.) in the band structure. The doping by field effect changes the distance between the Fermi level and the v.H.s.     

Does microstructure matter for statistical nanoindentation techniques?

In their paper, Trtik et al. (2009) identify spurious peaks in the application of statistical nanoindentation technique as a critical obstacle for mechanical phase identification. In this discussion, we show that Trtik et al.’s finding is a consequence of an unrealistic virtual 3-D checkerboard microstructure considered by the authors. These peaks are not a general feature of indentation on multiphase materials, nor can the presence of such peaks be attributed to an intrinsic shortcoming of the grid-indentation technique. We also show that the authors’ assertion of the absence of homogeneous material regions extending beyond 3 μm in cementitious materials is groundless.